Anodic coating of zinc base metals



- Patented Apr, 10, I934 UNITED STATES 1,954,000 'Asomc comma or me miss METALS Edward Cushman 'lruesdale and Ernest John Wilhelm, Palmerton, Pa., asalgnora to The New Jersey Zinc Company, New York, N. Y., a corporation or New Jersey No Drawing.

20Claims.

This invention relates to the coating of zinc base metals by anodic action, and has for its object the provision of an improved method producing useful anodic coatings on such metals,

5 as well as the provision, as new articles of commerce, of zinc base metals so coated. By zinc base metal we mean metals and alloys composed principally of zinc,

The invention is particularly concerned with m the production of anodic coatings on zinc and zinc base alloys for the purpose of protection, ornamentatiombr 0! serving as a base tor the application of paints, lacquers and the like so as to secure good adhesion'of the latter. Ordinary commercial zinc (in particular, rolled or strip zinc), zinc base alloys such as die-casting alloys containing over 90% zinc (for example, the alloys oi United States Patents Nos. 1,598,761, 1,663,215, 1,779,525, 1,852,434, 1,852,441 and 1,852,442), and wrought zinc base alloys (for example, the alloys of United States PatentsNos. 1,716,599, 1,832,653, 1,832,733 and 1,835,450) may be advantageously coated by the method of the invention. Numerous other zinc base alloys, particularly those containing small amounts of copper and/or aluminum. are likewise amenable to treatment in accordance with the invention.

In our copending application for Letters Patent oi the United States, Serial No. mobs, flied November 14, 1932, wehave disclosed the production oi-anodic coatings. on zinc irom strongly basic electrolytes, such as 0.2N to 1.0N

hydroxide solutions (pH value of about 13.3 or; more). 'In the course ot'our investigations. we

'have discovered that alkaline electrolytes of a slightly lower hydroxyl ion. concentration and containing a compound of anamphoteric metal yield anodicicoatings on zinc that are characteristically diilerent from those yielded by the strongly; alkaline electrolytes oi our aforementioned application. Alkaline solutions 01' the oxides or hydroxides of zinc, aluminum, beryllium.

and tin (or aqueous solutions of alkali zincates. aluminates, beryllates and stannates) are examples of such alkaline electrolytes containing amphoteric metal compounds. when the concentration of tree alkali exceeds a certain amount, in most casesabout 0.1N (pH about 13-), these amphoteric metal electrolytes behave essentially like simple solutions of an alkali hydroxide (e. g. sodium hydroxide) oi the same pH value, and yield dark coatings composed essentially of zinc oxide. If, however, the hydroxyl ion concentration of such alkaline solutions of these ampheteric metals be lowered appreciably below a pH Application November 14, an. Serial No. 842,011

value of 13 (the exact change of pH depending upon the concentration of ampholyte) the anodic coatings obtained on zinc are lighter in color, and may contain, in appropriate cases, small amounts of basic compounds of metals other than to zinc (e. g; aluminairom a sodium aluminate SOllltion). In the case of the true ampholytes, ii. the hydroxyl ion concentration is still further reduced, say into the region of pH values of 10 to 12, the anodic coatingsobtained iromalkaline solutions of these ampholytesbecomeloose, white and flocculent and consist mainly of the amphoterlc metal hydroxide. S uch anodic deposits are poorly adherent, and are 01 little, if any. p iactical value. I

Based on the foregoing discoveries, the method oi the present invention involves subjecting a zinc base metal, as anode, to electrolytic treatment in an alkaline electrolyte containing a compound oi an amphoteric metal and having a bydroxyl ion concentration suflicient to'yield a dense and adherent coating on the zinc anode while lnsufllc-ient to completely dominate the anion oi the ampholyte. Throughout this spectflcation and the appended claims, we include in so the class or amphoteric metals not only the true ampholytes, that is-to say, those metals that form oxides capable of acting as acids or bases without change of valence (e. g. tin, aluminum, beryllium'etcJ, but also those metalsiorming oxides which are soluble in both acids and bases, the metal, however, showing diilerent valences in the two solvents (e. g. chromium, tungsten etc); which metals may be desoribedaspseudo-ampholytes, The amphoteric metal electrolyte of the invention may be an aqueous solution'oi the salt formed by the combination of a strong base (such as the hydroxide of an alkali or alkalineearth metal) with the hydroxide or oxide of an amphoterlc metal (which reacts as a weak acid in such a combination) on account of hydrolysis, such a'solution in the case of the true ampholytes has a relatively high hydroxyl ion concentration approximating a pH value M313. 00 In accordance with the principle of the present 1 invention the hydroxyl ion concentration of the true amnhoteric metal electrolytes is preferably maintainedat a pH value 01' about 12 to about 13, although under certainconditions of operation and with certain pseudo-ampholytes, the pH value may be below 12 and may approximate neutrality. The concentration of the amphoteric metal compound present. in the electrolyte will be determined, to some extent, by the critical lim- 1m mentioned.

Since the anodic coating of the invention usually reproduces, in minute detail, the surface markings originally present on the zinc article, the appearance of the coating may be improved by bumng the zinc article before anodic treatment. This bufling procedure may advantageously be of the same character as used on zinc prior to electroplating. Other preliminary treatment, such as sand-blasting or etching which, like bufling, removes a surface film especially likely to be present on cast zinc surfaces,

,may be employed to facilitate the subsequent anodic treatment. Such preliminary surface treatment of the zinc article, may, of course, be dispensed with if desired.

The zinc article should be appropriately cleaned, prior to anodic treatment. This cleaning may be advantageously accomplished in a trisodium phosphate solution containing about six ounces per gallon of water (45 grams per liter of water). This solution is used hot as an electro-cleaner, with-the zinc article to be cleaned as the oathode, and with sufficient current to produce copious gassing. When used in this manner, grease (e. g. acquired during rolling or bufllng) and loose dirt are completely removed after one-half to three minutes of cleaning. After cleaning,

the zinc article is rinsed in hot and then in cold water to remove the residual cleaning solution;

' coating produced is firm, dense andadherent and varies in color depending upon the ampholyte used.

The following table gives the composition of several alkaline solutions of amphoteric metal hydroxides, and descriptions of the anodic coatings obtained therefrom by the practice of the invention:

Oonc. [roe Solution NaoH g g g zk Color cl coating Aluminum-.. zig sms per 3.15 A1101. White.

Do 4. per 131x110. Gray-white. Stannato pH 12.'s...---. son sr oran caromtoo more. Greenish-yellow.

The, anodic coatings of the invention not only protect the zinc or zinc alloy from corrosion but are also useful for decoration. These coatings may also be used as a base to increase the adherence of paint, enamel, lacquer etc. The light-colored anodic coatings produced in accordance with the invention may be given various colors by dyeing. The dyeing may take place during the electrolytic treatment by solution of the dye in the electrolyte or dyeing may be carried out by immersion of the coated article in an appropriate solution ofthe dye.

We claim:

7 1,954,000 I its of hydroxyl ion concentration hereinbefore 1. As a new article of commerce, a zinc base metal having a thin, dense and firmly adherent anodic coating containing a compound of an amphoteric metal other than zinc.

2. As a new article of commerce, a zincbase metal having a thin, dense and firmly adherent anodic coating containing a compound of zinc and a compound of another amphoteric metal.

3. As a new article of commerce, a zinc base metal having a thin, light-colored and firmly adherent anodic coating containing compounds of zinc and another amphoteric metal.

4. The method of producing a coating on a zinc-base metal which comprises subjecting the metal .as anode .to electrolytic treatment in an alkaline solution of an amphoteric metal compound (other than zinc) having a hydroxyl ion 'metal as anode to electrolytic treatment in an alkaline electrolyte containing a compound of an amphoteric metal and having a pH of 10 to 13. 5

6. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte containing a compound of an amphoteric metal other than zinc and having a pH of 12 to 13.

7. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic iu'eatment in an alkaline solution of an amphoteric metal hydroxide (or oxide) having a pH of 10 to 13.

8. The method of producing 'a coating on a zinc base metal-which comprises subjecting the metal as anode to electrolytic treatment in an alkaline solution of aluminum hydroxide (or oxide) having a pl-I of 10 to 13.

9. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline solution of tin hydroxide (or oxide) .having a pH of 10:to '13.

10. The metliod 'of. producing aacoating on a zinc base metal whichcomprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte containingaa/ compound of an amphoteric metal with a hydroxyl ion concentration adapted to produce on said zinc base metal a coherent coating containing a substantial amount of an insoluble compound of the amphoteric metal.

11. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline solution of an amphoteric metal hydroxide (or oxide) with a hydroxyl ion concentration adapted to produce on said zinc base metal a thin, dense and firmly adherent coating.

12. The method-of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline solution of an amphoteric metal with a hydroxyl ion concentration adapted to yield a dense and adherent-coating on the zinc anode while insufllcient to completely dominate the droxyl ion concentration adapted to produce on said zinc base metal an adherent coating.

14. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an aqueous solution of an alkali aluminate with a hydroxyl ion concentration adapted to produce by anodic action a firmly adherent coating on said zinc base metal.

15. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an aqueous solution of an alkali stannate with a hydroxyl ion concentration adapted to produce by anodic action a firmly adherent coating on said zinc base metal.

16. A method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in a solution selected from the group consisting of an alkaline solution of the oxides or hydroxides of zinc, aluminum, berylliumand tin; and an aqueous solution of alkali zincates, aluminates, beryllates and stannates; said solution having a hydroxyl ion concentration adapted to produce by anodic action a thin dense and firmly adherent coating on said zinc base metal.

17. A method of producing a coating on a zinc base metal which comprises subjecting the metal as'anode to electrolytic treatment in a solution of a compound of a metal that forms an oxide capable of acting as an acid or a base, said solution and acid oxides differing in valence, said solution having a hydroxyl ion concentration adapted to produce by anodic action a thin dense and firmly adherent coating on said zinc base metal.

19. A method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an amphoteric metal electrolyte consisting of an aqueous solution of a salt formed by the combination of a strong base with the hydroxide or oxide of an amphoteric metal, said solution having a hydroxyl ion concentration adapted to produce by anodic action a thin dense and firmly adherent coating on said zinc base metal.

20. A method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an aqueous solution of a salt selected from the group consisting of aluminates, stannates, chromates, tungstates, and zincates, said solution having a hydroxyl ion concentration adapted to produce by anodic action a thin dense and firmly adherent coating on said zinc base metal.

EDWARD CUSHMAN TRUESDALE. ERNEST JOHN WILHELM. 

